The present invention generally relates to indirect lighting fixtures of the type used in office, commercial and industrial environments, and more particularly it relates to lensed indirect lighting fixtures, that is, indirect lighting fixtures which achieve light distribution control, in part, by a prismatic lens or lenses located at the top opening of the fixture.
The advantages of indirect over direct lighting are well known and are discussed in U.S. Pat. No. 4,390,930 to Herst and Ngai, the applicants herein. This patent also discusses the incorporation of prismatic lenses into the optical design of indirect luminaires and, additionally, the psychological advantage of exposing at least a portion of the lens surface to the room so as to give people in an indirect lighting environment the sense of seeing an actual light source. The Herst patent discusses the offsetting problem that visible prismatic lenses, when illuminated, are normally uncomfortably bright to directly look at and proposes for desired aesthetic qualities and visual comfort an indirect luminaire having visible side lens strips, the surface brightness of which is carefully controlled to a low range of luminance. In terms of the luminaire's light distribution pattern, Herst shows that visual comfort can be achieved by a prismatic lens design (and lamp position) which intentionally directs a portion, but only a very small portion of light below the horizontal plane of the luminaire.
One problem with the lensed indirect luminaire disclosed in the U.S. Pat. No. 4,390,930 patent is that it tends however to be limited to mounting heights well above eye level, such as high wall mountings or ceiling suspended fixtures; this luminaire is not well suited for applications calling for low, near eye level mounting heights because at low mounting heights the lens, despite careful optical design, still becomes overly bright at high viewing angles from which the lens is normally seen. Exemplary of low mounting height uses are luminaires mounted to modular office furniture and, increasingly, fixtures mounted in proximity to word processing work stations. One type of fixture previously used in such applications is an indirect fixture having high opaque side walls and no refracting lens, the optical result being no significant lateral distribution of light. The advantage of such a totally indirect fixture design is that there are no overly bright lamp or lens surfaces to produce visual discomfort. The disadvantages, however, are that widespread distribution of the light is sacrificed and undersirable bright spots are produced on the overhead ceiling. Such fixtures also prevent persons from easily locating the fixture and realizing the resulting psychological advantages discussed above. Another indirect type of fixture designed for approximately eye level mounting heights is a fixture disclosed in applicant's co-pending application No. 046,970, wherein a lens cover is employed over the top opening of the fixture to spread the available source light laterally and more evenly. This fixture, however, does not control lens brightness and presents to an observer a visible lens that is uncomfortably bright.
The present invention overcomes the problem of providing low luminance in the visible lens surface of a lensed indirect luminaire mounted at low mounting heights. The invention increases light distribution control in vertical plane angles near the horizontal plane of the fixture, providing widespread light distribution from a lensed indirect luminaire having visible low brightness lens surfaces.